Delta subunit of human GABAA receptor

ABSTRACT

The present invention relates to the cloning of novel cDNA sequences encoding the α 4  and δ receptor subunits of the human GABA  A  receptor; to stably co-transfected eukaryotic cell lines capable of expressing a human GABA A  receptor, which receptor comprises at least one of the novel α 4  and δ receptor subunits; and to the use of such cell lines in screening for and designing medicaments which act upon the human GAGA A  receptor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 08/809,802, filed Jun. 19, 1997, now U.S. Pat. No. 6,455,276 which is PCT application PCT/GB95/02323 filed Sep. 9, 1995 entering the U.S. national phase.

FIELD OF THE INVENTION

This invention concerns the cloning of a novel cDNA sequence encoding a particular subunit of the human GABA_(A) receptor. In addition, the invention relates to a stable cell line capable of expressing said cDNA and to the use of the cell line in a screening technique for the design and development of subtype-specific medicaments.

BACKGROUND

Gamma-amino butyric acid (GABA) is a major inhibitory neurotransmitter in the central nervous system. It mediates fast synaptic inhibition by opening the chloride channel intrinsic to the GABA_(A) receptor. This receptor comprises a multimeric protein of molecular size 230–270 kDa with specific binding sites for a variety of drugs including benzodiazepines, barbiturates and β-carbolines, in addition to sites for the agonist ligand GABA (for reviews see Stephenson, Biochem. J., 1988, 249, 21; Olsen and Tobin, Faseb J., 1990, 4, 1469; and Sieghart, Trends in Pharmacol. Sci., 1989, 10, 407).

Molecular biological studies demonstrate that the receptor is composed of several distinct types of subunit, which are divided into four classes (α, β, γ and δ) based on their sequence similarities. To date, six types of a (Schofield et al., Nature (London), 1987, 328, 221; Levitan et al., Nature (London), 1988, 335, 76; Ymer et al., EMBO J., 1989, 8, 1665; Pritchett & Seeberg, J. Neurochem., 1990, 54, 862; Luddens et al., Nature (London), 1990, 346, 648; and Khrestchatisky et al., Neuron, 1989, 3, 745), three types of β (Ymer et al., EMBO J., 1989, 8, 1665), three types of γ (Ymer et al., EMBO J., 1990, 9, 3261; Shivers et al., Neuron, 1989, 3, 327; and Knoflach et al, FEBS Lett., 1991, 293, 191) and one δ subunit (Shivers et al., Neuron, 1989, 3, 327) have been identified.

The differential distribution of many of the subunits has been characterised by in situ hybridisation (Sequier et al., Proc. Natl. Acad. Sci. USA, 1988, 85, 7815; Malherbe et al., J. Neurosci., 1990, 10, 2330; Shivers et al., Neuron, 1989, 3, 327; and Wisden et al, J. Neurosci., 1992, 12, 1040) and this has permitted it to be speculated which subunits, by their co-localisation, could theoretically exist in the same receptor complex.

Various combinations of subunits have been co-transfected into cells to identify synthetic combinations of subunits whose pharmacology parallels that of bona fide GABA_(A) receptors in vivo (Pritchett et al., Science, 1989, 245, 1389; Malherbe et al., J. Neurosci., 1990, 10, 2330; Pritchett and Seeberg, J. Neurochem., 1990, 54, 1802; and Luddens et al., Nature (London), 1990, 346, 648). This approach has revealed that, in addition to an α and β subunit, either γ₁ or γ₂ (Pritchett et al., Nature (London), 1989, 338, 582; Ymer et al., EMBO J., 1990, 9, 3261; and Malherbe et al., J. Neurosci., 1990, 10, 2330) or γ₃ (Herb et al., Proc. Natl. Acad. Sci. USA, 1992, 89, 1433; Knoflach et al., FEBS Lett., 1991, 293, 191; and Wilson-Shaw et al., FEBS Lett., 1991, 284, 211) is also generally required to confer benzodiazepine sensitivity, and that the benzodiazepine pharmacology of the expressed receptor is largely dependent on the identity of the α and γ subunits present. Receptors containing a δ subunit (i.e. αβδ) do not appear to bind benzodiazepines (Shivers et al., Neuron, 1989, 3, 327). Combinations of subunits have been identified which exhibit the pharmacological profile of a BZ₁ type receptor (α₁β₂γ₂) and a BZ₂ type receptor (α₂β₁γ₂ or α₃β₁γ₂, Pritchett et al., Nature (London), 1989, 338, 582), as well as two GABA_(A) receptors with a novel pharmacology, α₅β₂γ₂ (Pritchett and Seeberg, J. Neurochem., 1990, 54, 1802) and α₆β₂γ₂ (Luddens et al., Nature (London), 1990, 346, 648). Although the pharmacology of these expressed receptors appears similar to that of those identified in brain tissue by radioligand binding, it has nonetheless not been shown that these receptor subunit combinations exist in vivo.

SUMMARY OF THE INVENTION

A combination of subunits comprising either the human α₄ GABA_(A) receptor subunit and/or the δ GABA_(A) receptor subunit has not hitherto been possible due to the non-availability of the human α₄ cDNA or human δ cDNA. This has consequently limited the use of cell lines in screening for subtype-specific medicaments, it being impossible to study the pharmacological profile of subunit combinations comprising the α₄ subunit and/or the δ subunit.

We have now ascertained the cDNA sequence of the α₄ subunit and the δ subunit of the human GABA_(A) receptor. These nucleotide sequences (SEQ ID NO:7 and SEQ ID NO:11), together with their deduced amino acid sequences (SEQ ID NO:8 and SEQ ID NO:12) corresponding thereto, are depicted in FIGS. 2 and 3 of the accompanying drawings.

The present invention accordingly provides, in a first aspect, a DNA molecule encoding the α₄ subunit of the human GABA_(A) receptor comprising all or a portion of the sequence (SEQ ID NO:7) depicted in FIG. 2, or a modified human sequence.

The present invention also provides, in another aspect, a DNA molecule encoding the δ subunit of the human GABA_(A) receptor comprising all or a portion of the sequence (SEQ ID NO:11) depicted in FIG. 3, or a modified human sequence.

The sequencing of the novel cDNA molecules in accordance with the invention can conveniently be carried out by the standard procedure described in accompanying Example 1; or may be accomplished by alternative molecular cloning techniques which are well known in the art, such as those described by Maniatis et al. in Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Press, New York, 2nd edition, 1989.

In another aspect, the invention provides a recombinant expression vector comprising the nucleotide sequence of the human GABA_(A) receptor α₄ subunit together with additional sequences capable of directing the synthesis of the said human GABA_(A) receptor α₄ subunit in cultures of stably co-transfected eukaryotic cells.

The present invention also provides a recombinant expression vector comprising the nucleotide sequence of the human GABA_(A) receptor δ subunit together with additional sequences capable of directing the synthesis of the said human GABA_(A) receptor δ subunit in cultures of stably co-transfected eukaryotic cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an expression vector in which R represents the nucleotide sequence of the α₄ or δ subunit of the GABA_(A) receptor, and the remainder of the expression vector is derived from the precursor vector pMSGneo.

FIG. 2 depicts the nucleotide sequence (SEQ ID NO:7) of the α₄ receptor subunit cDNA and the amino acid sequence (SEQ ID NO:8) of the encoded polypeptide.

FIG. 3 depicts the nucleotide sequence (SEQ ID NO:11) of the δ receptor subunit cDNA and the amino acid sequence (SEQ ID NO:12) of the encoded polypeptide.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides nucleotide sequences encoding the α₄ and δ subunits of the human GABA_(A) receptor, preparations of α₄ and δ receptor subunit proteins, preparations of receptors including α₄ or δ polypeptides, expression vectors including the nucleotide sequences, stably co-transfected eukaryotic cells and methods of their preparation and methods of screening for and designing medicaments which act upon the GABA_(A) receptor.

The term “expression vectors” as used herein refers to DNA sequences that are required for the transcription of cloned copies of recombinant DNA sequences or genes and the translation of their mRNAs in an appropriate host. Such vectors can be used to express eukaryotic genes in a variety of hosts such as bacteria, blue-green algae, yeast cells, insect cells, plant cells and animal cells. Specifically designed vectors allow the shuttling of DNA between bacteria-yeast, bacteria-plant or bacteria-animal cells. An appropriately constructed expression vector should contain: an origin of replication for autonomous replication in host cells, selective markers, a limited number of useful restriction enzyme sites, a high copy number, and strong promoters. A promoter is defined as a DNA sequence that directs RNA polymerase to bind to DNA and to initiate RNA synthesis. A strong promoter is one which causes mRNAs to be initiated at high frequency. Expression vectors may include, but are not limited to, cloning vectors, modified cloning vectors, specifically designed plasmids or viruses.

The term “cloning vector” as used herein refers to a DNA molecule, usually a small plasmid or bacteriophage DNA capable of self-replication in a host organism, and used to introduce a fragment of foreign DNA into a host cell. The foreign DNA combined with the vector DNA constitutes a recombinant DNA molecule which is derived from recombinant technology. Cloning vectors may include plasmids, bacteriophages, viruses and cosmids.

The recombinant expression vector in accordance with the invention may be prepared by inserting the nucleotide sequence of the GABA_(A) α₄ subunit or the GABA_(A) δ subunit into a suitable precursor expression vector (hereinafter referred to as the “precursor vector”) using conventional recombinant DNA methodology known from the art. The precursor vector may be obtained commercially, or constructed by standard techniques from known expression vectors. The precursor vector suitably contains a selection marker, typically an antibiotic resistance gene, such as the neomycin or ampicillin resistance gene. The precursor vector preferably contains a neomycin resistance gene, adjacent the SV40 early splicing and polyadenylation region; an ampicillin resistance gene; and an origin of replication, e.g. pBR322 ori. The vector also preferably contains an inducible promoter, such as MMTV-LTR (inducible with dexamethasone) or metallothionin (inducible with zinc), so that transcription can be controlled in the cell line of this invention. This reduces or avoids any problem of toxicity in the cells because of the chloride channel intrinsic to the GABA_(A) receptor.

One suitable precursor vector is pMAMneo, available from Clontech Laboratories Inc. (Lee et al., Nature, 1981, 294, 228; and Sardet et al., Cell, 1989, 56, 271). Alternatively the precursor vector pMSGneo can be constructed from the vectors pMSG and pSV2neo.

The recombinant expression vector of the present invention is then produced by cloning the GABA_(A) receptor α₄ subunit cDNA or the GABA_(A) receptor δ subunit cDNA into the above precursor vector. The receptor subunit cDNA is subcloned from the vector in which it is harboured, and ligated into a restriction enzyme site, e.g. the Hind III site, in the polylinker of the precursor vector, for example pMAMneo or pMSGneo, by standard cloning methodology known from the art, and in particular by techniques analogous to those described herein. Before this subcloning, it is often advantageous, in order to improve expression, to modify the end of the α₄ or δ subunit cDNA with additional 5′ untranslated sequences, for example by modifying the 5′ end of the α₄ or δ subunit DNA by addition of 5′ untranslated region sequences from the α₁ subunit DNA.

One suitable expression vector of the present invention is illustrated in FIG. 1 of the accompanying drawings, in which R represents the nucleotide sequence of the α₄ or δ subunit of the GABA_(A) receptor, and the remainder of the expression vector depicted therein is derived from the precursor vector pMSGneo.

According to a further aspect of the present invention, there is provided a stably co-transfected eukaryotic cell line capable of expressing a GABA_(A) receptor, which receptor comprises the alpha-4 receptor subunit, at least one beta receptor subunit and the delta receptor subunit.

In another aspect of the present invention, there is provided a stably co-transfected eukaryotic cell line capable of expressing a GABA_(A) receptor, which receptor comprises the alpha-4 receptor subunit, at least one beta receptor subunit and at least one gamma receptor subunit.

In a further aspect of the present invention, there is provided a stably co-transfected eukaryotic cell line capable of expressing a GABA_(A) receptor, which receptor comprises at least one alpha receptor subunit, at least one beta receptor subunit and the delta receptor subunit.

This is achieved by co-transfecting cells with three expression vectors, each harbouring cDNAs encoding for an α₄, β or δ GABA_(A) receptor subunit, or for an α₄, β or γ GABA_(A) receptor subunit, or for an α, β or δ GABA_(A) receptor subunit. In a further aspect, therefore, the present invention provides a process for the preparation of a eukaryotic cell line capable of expressing a GABA_(A) receptor, which comprises stably co-transfecting a eukaryotic host cell with at least three expression vectors, one such vector harbouring the cDNA sequence encoding the α₄ GABA_(A) receptor subunit another such vector harbouring the cDNA sequence encoding a beta GABA_(A) receptor subunit, and a third such vector harbouring the cDNA sequence encoding the delta GABA_(A) receptor subunit. The stable cell-line which is established expresses an α₄βδ GABA_(A) receptor.

The present invention also provides a process for the preparation of a eukaryotic cell line capable of expressing a GABA_(A) receptor, which comprises stably co-transfecting a eukaryotic host cell with at least three expression vectors, one such vector harbouring the cDNA sequence encoding the α₄ GABA_(A) receptor subunit another such vector harbouring the cDNA sequence encoding a beta GABA_(A) receptor subunit, and a third such vector harbouring the cDNA sequence encoding a gamma GABA_(A) receptor subunit. The stable cell-line which is established expresses an α₄βγ GABA_(A) receptor.

Similarly, the present invention provides a process for the preparation of a eukaryotic cell line capable of expressing a GABA_(A) receptor, which comprises co-transfecting a eukaryotic host cell with at least three expression vectors, one such vector harbouring the cDNA sequence encoding an alpha GABA_(A) receptor subunit, another such vector harbouring the cDNA sequence encoding a beta GABA_(A) receptor subunit, and a third such vector harbouring the cDNA sequence encoding the δ GABA_(A) receptor subunit. The stable cell line which is established expresses an αβδ GABA_(A) receptor.

Each receptor thereby expressed, comprising a unique combination of α₄, β and δ subunits, or α₄, β and γ subunits, or α, β and δ subunits, will be referred to hereinafter as a GABA_(A) receptor “subunit combination”. Pharmacological and electrophysiological data confirm that the recombinant α₄βγ receptor expressed by the cells of the present invention has the properties expected of a native GABA_(A) receptor.

Expression of the GABA_(A) receptor may be accomplished by a variety of different promoter-expression systems in a variety of different host cells. The eukaryotic host cells suitably include yeast, insect and mammalian cells. Preferably the eukaryotic cells which can provide the host for the expression of the receptor are mammalian cells. Suitable host cells include rodent fibroblast lines, for example mouse Ltk⁻, Chinese hamster ovary (CHO) and baby hamster kidney (BHK); HeLa; and HEK293 cells. It is necessary to incorporate the (α₄ subunit, at least one β and the δ subunit into the cell line in order to produce the required receptor, or alternatively the α₄ subunit and at least one β and one γ subunit or alternatively at least one α, one β and the δ subunit. Within this limitation, the choice of receptor subunit combination is made according to the type of activity or selectivity which is being screened for.

In order to employ this invention most effectively for screening purposes, it is preferable to build up a library of cell lines, each with a different combination of subunits. Typically a library of 5 or 6 cell line types is convenient for this purpose. Preferred subunit combinations include: α₄β₃γ₂, α₄β₃δ and α₆β₃δ. Another preferred subunit combination is α₄β₂γ₂.

As stated above, for each cell line of the present invention, three such vectors will be necessary, one containing the α₄ subunit, one containing a β subunit, and the third containing the δ subunit, or alternatively, one containing the α₄ subunit, one containing a β subunit, and the third containing a γ subunit, or alternatively, one containing an α subunit, one containing β subunit and one containing the δ subunit.

Cells are then co-transfected with the desired combination of three expression vectors. There are several commonly used techniques for transfection of eukaryotic cells in vitro. Calcium phosphate precipitation of DNA is most commonly used (Bachetti et al., Proc. Natl. Acad. Sci. USA, 1977, 74, 1590–1594; Maitland et al., Cell, 1977, 14, 133–141), and represents a favoured technique in the context of the present invention.

A small percentage of the host cells takes up the recombinant DNA. In a small percentage of those, the DNA will integrate into the host cell chromosome. Because the neomycin resistance gene will have been incorporated into these host cells, they can be selected by isolating the individual clones which will grow in the presence of neomycin. Each such clone is then tested to identify those which will produce the receptor. This is achieved by inducing the production, for example with dexamethasone, and then detecting the presence of receptor by means of radioligand binding.

In a further aspect, the present invention provides protein preparations of GABA_(A) receptor subunit combinations, especially human GABA_(A) receptor subunit combinations, derived from cultures of stably transfected eukaryotic cells. The invention also provides preparations of membranes containing subunit combinations of the GABA_(A) receptor, especially human GABA_(A) receptor subunit combinations, derived from cultures of stably transfected eukaryotic cells.

The cell line, and the membrane preparations therefrom, according to the present invention have utility in screening and design of drugs which act upon the GABA_(A) receptor, for example benzodiazepines, barbiturates, β-carbolines and neurosteroids. The present invention accordingly provides the use of the cell line described above, and membrane preparations derived therefrom, in screening for and designing medicaments which act upon the GABA_(A) receptor. Of particular interest in this context are molecules capable of interacting selectively with GABA_(A) receptors made up of varying subunit combinations. As will be readily apparent, the cell line in accordance with the present invention, and the membrane preparations derived therefrom, provide ideal systems for the study of structure, pharmacology and function of the various GABA_(A) receptor subtypes.

The following non-limiting Examples illustrate the present invention.

EXAMPLE 1

Isolation and Sequencing of cDNAs Encoding the Human GABA_(A) Receptor α₄ Subunit

a) cDNA Libraries

cDNAs were cloned from human foetal brain and adult hippocampus cDNA libraries. All cDNA libraries were constructed in the lambdaZAP vector, and were purchased from Stratagene (San Diego, Calif.). For screening, the cDNA libraries were plated according to the manufacturer's instructions, at 40,000 pfu per 137 mm plate. Filter lifts were taken using Hybond N filters (Amersham) according to the manufacturer's instructions.

b) Isolation of cDNA Encoding Human α₄ Subunit

A human α₄ probe was first generated by polymerase chain reaction (PCR) using oligonucleotide primers (synthesised on an Applied Biosystems 380B synthesizer) derived from the bovine α₄ sequence (Ymer et al, FEBS Lett., 1989, 258, 119): 5′TTTCAGGAATTCCAGTGCTGAGAGAAAAGCATCCTGAAAC3′ (bp 1121–1160, containing an EcoRI restriction enzyme site) SEQ. ID. NO.:1, and 5′ATCCAGAAGCTTGTGGAGCAGAGGGAGTAGTAGTGGC3′ (antisense, bp 1540–1577, incorporating a HindIII restriction enzyme site) SEQ. ID. NO.:2. PCR was performed as described, for example, by Whiting et al in Proc. Natl. Acad. Sci., USA, 1990, 87, 9966, using a human foetal brain cDNA library as a template. The PCR product was digested with EcoRI and HindIII and subcloned into similarly digested pBluescript SK- and its identity confirmed by DNA sequencing using standard techniques and the Sequenase II enzyme (United States Biochemicals).

A human foetal brain cDNA library was screened using ³²P labelled human α₄ probe DNA as described above. A single cDNA clone, approximately 2500 bp, was obtained. DNA sequencing indicated that this cDNA clone contained 3′ untranslated sequences and 3′ coding region up to bp 1162 of the bovine cDNA sequence. The missing 5′ sequence was obtained by anchored PCR using human brain 5′-RACE-Ready cDNA (CLONTECH, Palo Alto, Calif.), according to the manufacturer's instructions. The antisense oligonucleotides used for nested PCR were 5′ATTGGCATTTGTATTCTGCAGAGG3′ SEQ. ID. NO.:3, and 5′GGAAGATTTGCTTGAATGGTTTGG3′ SEQ. ID. NO.:4. A 1200 bp PCR product was obtained. DNA sequencing confirmed that this cDNA contained the missing 5′ sequence of the α₄ cDNA, extending to 130 bp 5′ of the initiating ATG codon.

A full length α₄ cDNA was generated by PCR using oligonucleotide primers generated from sequences of the 5′ and 3′ untranslated region: 5′ sense primer 5′CCTGGATCCGTGAACAGGCTTGAAGTATG3′ (incorporating a BamHI restriction enzyme site) SEQ. ID. NO.:5; 3′ antisense primer 5′ACGAATTCACATTAGACTTTCTGATTTCTC3′ (incorporating an EcoRI restriction enzyme site) SEQ. ID. NO.:6. PCR was performed using human brain thalamus cDNA. A 1500 bp product was generated which was subcloned into the cloning/eukaryotic expression vector pcDNA/Amp (Invitrogen). The cDNA was sequenced completely on both strands using an Applied Biosystems 373A DNA sequencer and dye terminator chemistry according to the manufacturer's instructions.

The complete nucleotide sequence of the cDNA encoding the human α₄ subunit, together with the deduced amino acid sequence corresponding thereto is shown in FIG. 2 of the accompanying drawings SEQ. ID. NOS.:7 and 8.

EXAMPLE 2

Isolation and Sequencing of cDNAs Encoding the Human GABA_(A) Receptor δ Subunit

a) cDNA Libraries

As described in Example 1(a).

b) Isolation of cDNA Encoding Human δ Subunit

A rat δ subunit probe was first generated by PCR using oligonucleotide primers derived from the rat δ subunit sequence (Shivers et al, Neuron, 1989, 3, 327): 5′AGCCCGAATTCCATGGACGTTCTGGGCTGGCTG3′ (bp 18–51, incorporating an EcoRI restriction enzyme site) SEQ. ID. NO.:9 and 5′GGTTTCCAAGCTTACTTTGGAGAGGTAGC3′ (bp 1357–1390, incorporating a HindIII restriction enzyme site) SEQ. ID. NO.:10. PCR was performed as described, for example, by Whiting et al, Proc. Natl. Acad. Sci., USA, 1990, 87, 9966, using rat brain cDNA as template. A 1400 bp product was obtained, subcloned into pBluescript SK- and its identity confirmed by DNA sequencing. A human hippocampus cDNA library was screened using ³²P labelled rat δ subunit probe DNA as described above. A single clone was obtained containing an 1800 bp insert. DNA sequencing indicated that this cDNA contained the complete coding region of the human δ subunit. The cDNA was sequenced completely on both strands using an Applied Biosystems 373A DNA sequencer and dye terminator chemistry according to the manufacturer's instructions.

The complete nucleotide sequence of the cDNA encoding the human δ subunit, together with the deduced amino acid sequence corresponding thereto is shown in FIG. 3 of the accompanying drawings SEQ. ID. NOS.:11 and 12.

EXAMPLE 3

Expression of Human α₄ cDNA in Xenopus Oocytes

The human α4 cDNA (Example 1, FIG. 2) was subcloned into the eukaryotic expresion vector, pcDNA I Amp (Invitrogen, San Diego Calif.). Expression of this cDNA was investigated using the Xenopus oocyte system. Methods for preparation of Xenopus oocytes, nuclear injection of cDNAs, and eletrophysiological recordings from oocytes expressing recombinant GABA_(A) receptors, are well documented (see, for instance, Hadingham et al., Mol. Pharmacol., 1993, 44, 1211–1218).

When co-expressed with β₂ and γ₂ cDNAs (Hadingham et al., Mol. Pharmacol., 1993, 44, 1211–1218) minimal expressed of GABA_(A) gated chloride currents were observed (10–50 nA whole cell currents as measured under voltage clamped conditions). To increase the efficiency of expression the α₄ cDNA was re-engineered so as to replace the 5′ untranslated sequence and signal peptide with the corresponding α₁ sequence. PCR was performed using the α₁ cDNA (Schofield et al., Nature (London), 1987, 328, 221) as template. Primers were (i) 5′TAATGAGTTTAAACCATAGCTTCTTCCAGT3′ (bp 12–35 of α₁ incorporating a BamHI site) SEQ. ID. NO.:11, and (ii) 5′CATGATGGATCCGCCCGCTCAGAC3′ (bp 269–305 incorporating a PmeI site) SEQ. ID. NO.:12. The BamHI-PmeI cut PCR fragment was subcloned into similarly cut α₄ pCDNA I Amp. When this α₄ construct was co-expressed in Xenopus oocytes with β₂ and γ₂ cDNAs robust GABA_(A) gated currents of up to 1000 nA whole cell current were obtained. 

1. An isolated DNA molecule comprising the nucleotide sequence encoding a δ subunit of a human GABA_(A) receptor provided by SEQ ID NO
 12. 2. An isolated stably co-transfected eukaryotic cell line expressing a human GABA_(A) receptor comprising cDNA encoding the δ receptor subunit of SEQ ID NO: 12, cDNA encoding at least one a receptor subunit, and cDNA encoding at least one β receptor subunit.
 3. The cell line of claim 2, wherein said cell line is a rodent fibroblast cell line.
 4. A process for the preparation of an isolated eucaryotic cell line expressing a human GABA_(A) receptor comprising stably co-transfecting a eukaryotic host cell with at least one recombinant expression vector comprising a human cDNA sequence encoding the δ receptor subunit of SEQ ID NO: 12, at least one recombinant expression vector comprising a human cDNA sequence encoding an a receptor subunit, and at least one recombinant expression vector comprising a human cDNA sequence encoding a β receptor subunit.
 5. The process of claim 4, wherein said cell line is a rodent fibroblast cell line.
 6. A recombinant expression vector comprising a the nucleotide sequence encoding a δ subunit of a human GABA_(A) receptor provided by SEQ ID NO: 12 together with additional sequences capable of directing the synthesis of said δ receptor subunit in a culture of isolated stably co-transfected eukaryotic cells.
 7. An isolated protein preparation of human GABA_(A) receptor derived from a culture of eukaryotic cells stably transfected with cDNA encoding a human GABA_(A) receptor, wherein said GABA_(A) receptor has a subunit combination that includes the human δ receptor subunit of SEQ ID NO: 12, provided that said culture of eukaryotic cells does not endogenously express said human GABA_(A) receptor.
 8. The protein preparation of claim 7, wherein said subunit combination is selected from the group consisting of α₄β₃δ and α₆β₃δ.
 9. An isolated membrane preparation wherein the membrane preparation comprises a human GABA_(A) receptor comprising the human δ receptor subunit of SEQ ID NO:12, derived from an isolated culture of eukaryotic cells stably transfected with cDNA encoding a human GABA_(A) receptors wherein said GABA_(A) receptor has a subunit combination that includes the human δ receptor subunit of SEQ ID NO: 12, provided that said culture of eukaryotic cells does not endogenously express said human GABA_(A) receptor.
 10. The membrane preparation of claim 9, wherein said subunit combination is selected from the group consisting of α₄β₃δ and α₆β₃δ. 